Field of the Invention
Embodiments of the invention relate to the field of circuit protection devices. More particularly, the present invention relates to a surge protection device including a metal oxide varistor stack with an integral thermal disconnect configured to provide an expedient thermal response in the event of overheating due to an abnormal overvoltage condition.
Discussion of Related Art
Overvoltage protection devices are used to protect electronic circuits and components from damage due to overvoltage fault conditions. These overvoltage protection devices may include metal oxide varistors (MOVs) that are connected between the circuits to be protected and a ground line. MOVs have a unique current-voltage characteristic that allows them to be used to protect such circuits against catastrophic voltage surges. These devices may utilize a thermal link which melts during an overvoltage condition to form an open circuit. In particular, when a voltage that is larger than the nominal or threshold voltage of an MOV is applied to the device, current flows through the MOV which generates heat that causes the thermal link to melt. Once the link melts, an open circuit is created which prevents the overvoltage condition from damaging the circuit to be protected. However, these existing circuit protection devices do not provide an efficient heat transfer from the MOV to the thermal link, thereby delaying response times. In addition, MOV devices have relatively high inductance characteristics which degrade performance in the presence of fast overvoltage transients. Moreover, existing circuit protection devices are complicated to assemble and connect in certain applications such as, for example, in LED protection which increases manufacturing costs. Accordingly, it will be appreciated that improvements are desirable in present day circuit protection devices employing metal oxide varistors.